1. Field of the Invention
The present invention relates to a nonaqueous secondary battery, a positive-electrode active material and a process for producing the same. More particularly, it relates to a nonaqueous secondary battery having a negative electrode comprising an active material which contains lithium or enables lithium to intercalate/deintercalate or insertion/desertion, and a positive electrode comprising an active material which contains a lithium manganese composite oxide, and to a process for producing the positive-electrode active material.
2. Description of the Prior Art
With downsizing and energy saving of electronic devices, secondary batteries utilizing alkali metals such as lithium have been attracting more attention.
Where an alkali metal such as lithium alone is employed for the negative electrode of a secondary battery, a dendrite (branching tree-like crystal) appears and grows on the surface of an active material during a repeated charging and discharging process, i.e., during metal dissolution-deposition process. As a result, the dendrite may penetrate through a separator of the secondary battery and contact the positive electrode. This will result in a short circuit within the battery.
Further, if an alkali metal alloy is employed for the negative electrode, the growth of the dendrite can be suppressed, as compared with the case of the alkali metal alone, so that the charge-discharge cycle characteristics are improved. However, the generation of the dendrite on the surface of the active material cannot be completely prevented, and there still exists the problem of the short circuit within the secondary battery.
In recent years, carbon materials and organic materials (e.g., conductive polymers) for utilizing an alkali metal ion absorption-releace process have been employed for a negative electrode instead of alkali metals and alloys thereof for utilizing a dissolution-deposition process or dissolution-deposition diffusion in solid process. The use of these materials makes it possible to avoid the generation of the dendrite because of the operational principle, resulting in a remarkable decrease in short circuits within batteries.
For positive electrode, on the other hand, there have been attempts to employ vanadium pentaoxide, titanium disulfide, tungsten trioxide, manganese dioxide, lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2), spinel-type lithium manganese oxide (LiMn.sub.2 O.sub.4) and the like. Vanadium pentaoxide used for the positive electrode of a secondary battery serves as an active material in a charged state immediately after the synthesis thereof, as disclosed, for example, in Japanese Unexamined Patent Publications No. 61-116758 (1986), No. 61-116757 (1986), No. 1-246763 (1989) and No. 3-105858 (1991). In these patent applications, it is disclosed that a lithium secondary battery presenting a high voltage and having an improved charge-discharge cycle life can be obtained by using for the positive electrode an amorphous vanadium pentaoxide prepared by way of quick quenching.
Further, in Japanese Unexamined Patent Publications No. 55-136131 (1980), No. 62-90863 (1987), No. 62-25637 (1987), No. 62-264560 (1987), No. 3-64860 (1991), No. 4-28162 (1992) and No. 4-155775 (1992), it is disclosed that lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2) or a composite oxide thereof, or a composition comprising any of such composite oxides and another element added thereto can be used as a positive-electrode active material presenting a high potential. Still further, it is proposed to obtain a lithium secondary battery presenting a high voltage by using an electrode containing such positive-electrode active materials. However, raw materials of these positive- electrode active materials are expensive and, hence, the production of batteries will be costly.
On the other hand, in Japanese Unexamined Patent Publications No. 63-114064 (1988), No. 63-114065 (1988), No. 1-294362 (1989), No. 1-307159 (1989), No. 3-67464 (1991), No. 3-108262 (1991), No. 3-110757 (1991) and No. 3-110758 (1991), there are disclosed composite oxides comprising relatively inexpensive manganese dioxide, manganese dioxide composition containing lithium or a composite oxide including manganese dioxide and another element added thereto. These substances each serve as an active material in a charged state only immediately after the synthesis thereof. Therefore, in the case that a lithium intercalation/deintercalation carbon material, graphite or the like which is in an initial discharged state is used as a negative-electrode active material in combination with the aforesaid positive-electrode active material, it becomes necessary to discharge the positive electrode or to charge the negative electrode in the initial stage, thereby necessitating an additional step in the battery production process.
Further, a spinel-type lithium manganese oxide (LiMn.sub.2 O.sub.4) and a composite oxide comprising the same and another element added thereto are disclosed in T. Ohsuku, M. Tagawa, T. Hirai, J. Electrochem. Soc., 137, 769 (1990), Japanese Unexamined Patent Publications No. 2-270268 (1990), No. 3-108261 (1991), No. 4-14757 (1992), No. 4-141954 (1992) and No. 4-147573 (1992). Although the aforementioned problems can be overcome by using these composite oxides, there still exist problems that the potential thereof is so high as to cause easy decomposition of electrolytic solution and material, and that only low charge-discharge capacity is expected.